For many years, telephones have had keypads instead of rotary dials for entering the destination telephone number. The caller enters digits by pressing keys or buttons on this keypad. Each button causes a unique pair of tones to be sent from the telephone to the network. Each unique tone pair is recognized by the network as a different number.
FIG. 2 illustrates a typical telephone keypad wherein each button generates unique tone pairs for each number. There are seven frequency lines disposed in a four by three matrix for the twelve buttons. As each button is pushed, the pair of frequencies are activated, thus generating a unique dual-tone, multifrequency (DTMF) signal. The frequencies were selected so that they are not harmonic overtones of each other. For example, pressing the button "5" activates the frequency pair 770 Hz and 1336 Hz, which a network or other monitoring device recognizes as a "5".
In addition to representing numeric digits, some of the buttons also are used to represent non-numeric characters. There are 3 letters associated with eight of the numbered buttons (2-9), representing the English alphabet (no Q or Z). In recent years, businesses have discovered that by forming telephone numbers using the letters on the buttons, they can create telephone numbers that were easy to remember, and consumers are more likely to call a business whose number they can easily remember. Thus, vanity numbers became a big business. Many businesses now have telephone numbers that include, in whole or part, some alphabetic form of their names or slogans. In fact, mnemonic names are now used as a telephone number for access services such as, for example, making a collect call by dialing 1-800-OPERATOR.
A problem for some consumers using these vanity numbers is caused by the fact that the letters are difficult to find. Three letters are found on each of eight buttons; they are usually much smaller in size than the numeral, and they are arranged in groups not commonly associated with each other. As a result, there is a problem in locating the required letters, resulting in dialing difficulty, delay, and frequent misdialing.